The maximum speed at which an aircraft may be routinely flown is limited by
the ability of the structure to support the resulting aerodynamic loads and by the compressibility effects encountered when flying close to the local speed of sound.
At low altitudes the calibrated airspeed producing limiting aerodynamic loads is the limiting value. This is called VMO.
The mach number at which compressibility effects become the limiting value is termed MMO. As altitude is increased, the local speed of sound decreases, thereby reducing the CAS equating to MMO. At high altitudes VMO is greater than MMO and at lower altitudes MMO is greater than VMO.
So VMO is the limiting speed at low altiude and MMO is the limiting speed at high altitude (as stated in two previous posts).
A mach meter produces an indication of mach number based on the ratio of dynamic pressure to static pressure. It does this by taking in pitot pressure and static pressure. An ASI differential capsule is then used to subtract static pressure from pitot pressure to leave dynamic pressure.
Movement of this capsule is then modified, using an altimeter aneroid capsule which senses static pressure. The mechanism is arranged such that the resultant output motion represents dynamic pressure divided by static pressure.